1. Field of the Invention
The present invention generally relates to an arrangement for and a method of driving a piezoelectric motor to oscillate a mirror, especially for use in a color image projection system operative for projecting a two-dimensional image in color while maintaining low power consumption, high resolution, miniature compact size, quiet operation and minimal vibration.
2. Description of the Related Art
It is generally known to project a two-dimensional image on a screen based on a pair of scan mirrors which oscillate in mutually orthogonal directions to scan a laser beam over a raster pattern. However, the known image projection systems project the image with limited resolution, typically less than a fourth of video-graphics-array (VGA) quality of 640×480 pixels, and not in true color.
It is also known to oscillate one of the scan mirrors by inertial forces, such as taught by U.S. patent application Ser. No. 10/975,888, filed Oct. 27, 2004, and U.S. patent application Ser. No. 10/387,878, filed Mar. 13, 2003, both assigned to the assignee of the instant application and both incorporated herein by reference, by connecting a piezoelectric motor comprising a pair of piezoelectric transducers to a frame at opposite sides of a hinge about which the scan mirror is free to oscillate. A periodic drive signal is applied to both transducers to cause one transducer to extend and push against one side of the frame, while the other transducer is simultaneously caused to contract and pull against the opposite side of the frame, and vice versa. The push-pull forces are alternately transmitted through the frame to cause the scan mirror to oscillate about the hinge at a mechanical resonance frequency. A light beam incident on the mirror is swept as one or more scan lines in space.
A drive circuit for generating the periodic drive signal typically requires a relatively high AC voltage, e.g., 50 volts peak-to-peak, with a drive or fundamental frequency precisely tuned to the mechanical resonance frequency of the motor. One known drive circuit uses a high DC voltage, e.g., 50 volts, and then passes this voltage through a linear control. However, this drive circuit incurs relatively high power losses and is inefficient.
Other known drive circuits involve using energy recycling. For example, the piezoelectric transducer has an inherent capacitance, and an inductor having an inductance is connected to the transducer to form a resonant circuit. A low DC drive voltage, e.g., 5 volts, is switched at the electrical resonance frequency created by the resonant circuit and is used to create a voltage boost to the drive voltage on the order of 50 volts AC. The drive frequency must therefore match the electrical resonance frequency in order to achieve drive voltage boosts on the order of ten times.
Experience has shown, however, that such energy recycling drive circuits are unsatisfactory in practice. The capacitance of the piezoelectric transducer varies from unit to unit, and also with temperature and with age. The inductance of the inductor also varies from unit to unit, and also with temperature and with age. By way of example, for a temperature variation of 40° C., the capacitance can change by 20%; the inductance can change by 5%; and the electrical resonant frequency can change by 12%.
To counter such undesired variation in the electrical resonant frequency, the prior art has proposed sophisticated, complex compensation circuitry to compensate for such capacitance and inductance changes. This compensation circuitry uses additional capacitors, additional inductors, switches and logical controls, all of which add both cost and size to the overall arrangement, as well as additional components whose impedances are also dependent on tolerances, temperature and ageing.
Still other known drive circuits use manual adjustments to tune the electrical resonant frequency. Typically, a mechanically tunable inductor is adjusted. However, this drive circuit is unsatisfactory since a single manual adjustment cannot adequately compensate over a whole range of tolerance, temperature and ageing variations.